Here are some good ways to kill frost trolls:
Combat
Use a shield to block the frost troll's attacks. You can also stagger them by pressing attack while holding block.
Weakness
Since frost trolls rely to the cold, use flames on the frost trolls. You could use a flame sword, a flame spell, the fire breath shout, or summon a flame atronatch to kill it. It's that simple.
Extra Help
If you still can't fight, get some followers! Use some of the tools above to help kill the frost troll. Make sure your follower has a weapon with the flame affect or if he or she's a mage cast flame spells, or even a flame staff!
Extra Feature
There is a book that tells you how to kill frost trolls and how to protect yourself against them. It's called How to Kill a Frost Troll. It'll give you more information on how to defend yourself and fight frost trolls.